I'm currently designing a keep for my players to explore. It will be a ruined keep, taken over by an evil cult. But to start off the design, I am looking at how it would have been designed in its prime. Here are some rooms I've thought of when creating the layout.
One thing that should be larger and more prominent is large food and water storage areas. Castles, during sieges, were designed to not only house and feed the normal staff but the subjects from outlying farms as well.
It could be a place to add some details.
Eg: Huge underground cave that was turned into a natural water cistern but is now filled with something else...
May or may not be an actual room in a castle, but there's often going to be one or multiple cesspits. This could literally be simply a pit under a garderobe/bathroom or it could be a walled and enclosed space, but if present it would be serviced regularly by gong farmers.
More detail: The cesspits will usually be placed outside the walls (and away from the water storage) of the castle and the garderobes were often placed in the outer walls with a shaft leading to the outside of the castle.